A variety of methods are known for making optical fiber preforms in the manufacture of optical fiber including, for example, Modified Chemical Vapor Deposition (MCVD), Sol-Gel, and Vapor Axial Deposition (VAD). In the VAD method soot preforms are prepared by reacting glass precursors in an oxyhydrogen flame to produce silica particles. The silica particles are deposited on a starting rod. The starting rod is slowly pulled upward while it is rotated, and the silica particles are deposited axially on the rod as it is pulled. Very large, and long, soot preforms can be fabricated. Typically the soot for the core is produced by a core torch and the soot for the cladding by a cladding torch. In this way, the composition of the glass can be varied from the center portion of the perform to the outside portion. Variation in glass composition is required for providing the refractive index difference necessary to produce light guiding in the optical fiber. After the soot is deposited, the preform is heated to consolidate the silica particles into a solid transparent glass body. Optical fiber is manufactured by drawing fiber from the consolidated preform using a conventional fiber drawing apparatus.
It has been recognized that the main functional part of an optical fiber is the core and the inner cladding. This part of the fiber carries 99+% of the optical energy. However, it typically consists of but 5% of the mass of the optical fiber. Accordingly, state of the art manufacture often makes use of an inner portion constituting core and inner clad region fabricated by soot deposition using MCVD or VAD, then overcladding the core rod with a material of less demanding properties. Consequently, the overcladding—the bulk of the preform—may be produced by less costly processing. Overcladding may entail direct deposition on the core rod, or may result from using a separate “overcladding tube”. Such overcladding tubes have been produced from soot or fused quartz.
For uniform lightguiding properties in the optical fiber pulled from VAD preforms it is important that the preform dimensions are precise and uniform. This includes the diameter of the core and the diameter of the cladding, i.e. the preform cross section, as well as the overall preform diameter.
In typical VAD processes, the position of the tip, and the growth rate at the tip, determines the rate of pulling of the preform. If the tip temperature changes unexpectedly, e.g. decreases unexpectedly, the z-direction growth rate of the core increases. At the same time deposition of the cladding soot stays constant. This results in less cladding soot per z-direction increment, i.e. a thinner cladding than the process design calls for. The same risk prevails for the cladding soot. If the rate of deposition of the cladding soot increases (or decreases) unexpectedly, the displacement monitor at the tip does not detect that change. Again the cladding diameter to core diameter ratio changes without detection.